Substituting microalgae with fecal sludge for biohythane production enhancement and cost saving through two-stage anaerobic digestion

Finding a low-cost, simple technology for producing high-quality bioenergy (e.g., biohythane: bioH2 and bioCH4) from waste biomass is challenging. Production of cleaner energy, such as biohythane, than methane from microalgae (MA), is a suitable approach. However, having a low C-N ratio of microalgae limits scaling up the process. Therefore, this study took the opportunity to use fecal sludge (FS) as a co-substrate for C-N ratio optimization to improve biohythane production while reducing cost. The biomass hydrothermal pretreatment enhanced soluble chemical oxygen demand (sCOD) from 390 mgL−1 to 1085 mgL−1, resulting volatile fatty acid (VFA) accumulation rate of 1853 mgL−1 at a mixing ratio of 50:50. The results of two-stage anaerobic digestion (TSAD) of 50-50 MA-FS showed a cumulative biohydrogen yield of 24.5 mLg−1-VS (Volatile Solid) and biomethane yield of 281 mLg−1-VS with a synergetic effect of 52.7%. The techno-economic analysis suggests a return on investment (ROI) of 4.8 for utilizing a 50-50 biomass ratio. In addition, the input cost could be reduced to 75.6 % if the ratio of MA and FS was adjusted to 25–75. Therefore, the TSAD of MA and FS is a promising option to enhance the biohythane yield and its high economic potential.